Project update:
Ended up cutting down the CPU cooler backplate to get it to fit, made sure to sand any sharp edges and clean up burrs/particulate before mounting. Both of the drives on the bottom tray are in, although they're sort of hanging on the mounting screws in a way that I don't like much. The screws don't seem to go in far enough to tighten the drives to the tray so there's a few MM of slip. I guess I could cut down the screws or find shorter ones or maybe get some washers in there?
It did post on the first try and all the diagnostics came back clean, win10 pro installed, and now things get interesting.
I'm waiting on some aluminum sheet to craft a tray for the top drive or two and will need to do some additional bodywork on the rear case panel to get the USB cables needed for the 3rd/4th drives routed back inside the case. Also ordered a second stick of RAM; decided that for another $25 or so it was a reasonable add.